The wordress comparison is a bit off when calling it a "webapp upgrade", because in that scenario you're using yourself, a server administrator, to mean "the customer." In that scenario, you're installing software onto your computer so it is exactly like a fat app.
If you look from the perspective of your readers (also wordpress "customers") they received the wordpress update without doing any work or restarting anything. Their experience could be like that of Google Docs.

One of the things I like most about Google's Chrome web browser is how often it is updated. But now that Chrome has rocketed through eleven versions in two and a half years, the thrill of seeing that version number increment has largely worn off. It seems they've picked off all the low hanging ...